A honeycomb structure body is used as a filter which traps a particulate matter included in an exhaust gas from an internal combustion engine, a boiler or the like, a catalyst carrier of a catalyst for purifying the exhaust gas, or the like. In particular, a honeycomb structure body made of a ceramic material has excellent characteristics such as resistance to heat, resistance to thermal shock and resistance to oxidation, and is broadly used suitably for the above-mentioned application.
In general, the honeycomb structure body made of the ceramic material is manufactured by kneading ceramic powder with an organic binder, water and the like to form clay, forming the clay into a honeycomb shape by an extrusion forming process or the like, subjecting the resultant material to drying, calcining and the like if necessary, and then firing the material. However, in such manufacturing steps, various defects (cut, hole, crack, attachment of dust, dirt, uneven surface, scratch, mixing of foreign matters, and the like) are generated sometimes in the outer wall of the honeycomb structure body.
Such defects of the outer wall deteriorate the strength of the honeycomb structure body, a filtering performance in a case where the honeycomb structure body is used as the filter, an exhaust gas purification performance in a case where the honeycomb structure body is used as the catalyst carrier, and the like. Therefore, the presence of the defect in each of manufactured honeycomb structure bodies needs to be inspected. Heretofore, such an inspection of the defect generated in the outer wall of the honeycomb structure body has been performed by a visual inspection (any prior art document concerning such a conventional technology is not especially found).
However, such a visual inspection is a sensory inspection based on a human visual sense, and hence the defect is missed in not a few cases. Moreover, fluctuation is generated in a defect detection level owing to a difference in a visual power or a fatigue degree, and it is difficult to perform stable inspection. Furthermore, fluctuation is generated in an inspection time per honeycomb structure body owing to the difference in the fatigue degree or a product dimension, and it is therefore difficult to perform the efficient and stable industrial production of the honeycomb structure body. In addition, when the defect detection level has to be raised, the number of inspecting persons has to be increased, and the inspection time per honeycomb structure body has to be lengthened, so that productivity deteriorates.